It is known that the components of the high temperature sections of turbines and relative accessories are protected with a thermal barrier coating, both when they are parts of aeronautical engines and when they are inside gas turbines intended for the production of energy.
In general, a thermal barrier coating has a multilayer structure. Typically, a thermal barrier coating comprises a definitive barrier layer, also called top coat, arranged on a connecting layer, also called bond coat, which, in turn, is arranged immediately above the substrate to be protected.
FIG. 1 schematically illustrates a detail of an article provided with a thermal barrier coating TBC of known type. A substrate S is defined by a portion of a component to be protected, for example a component of a turbine. A bond coat BC is arranged immediately on the substrate S. The bond coat BC generally comprises metallic materials such as cobalt, nickel, chromium, aluminium, yttrium, etc.
The bond coat BC is applied to the substrate S by means of an appropriate known deposition technique such as, for example, thermal spray, vacuum plasma spray (VPS), air plasma spray (APS), etc.
The structure and roughness of the outermost surface of the bond coat BC depend, in general, on the deposition technique and the powder used.
Referring again to FIG. 1, a top coat TC is lastly arranged on the bond coat BC and defines below, with the latter, an interface I.
The top coat TC typically comprises a ceramic material, for example yttrium-stabilised zirconia, and is, in turn, applied on the bond coat BC by means of an appropriate known deposition technique, typically by APS or by Electron Beam Physical Vapour Deposition (EB-PVD).
It is known, for example from U.S. Pat. No. 5,403,669, that aluminium is diffused inside the bond coat BC before proceeding with deposition of the top coat TC, in order to increase resistance to the highly oxidising environments of the thermal barrier coating as a whole.
FIG. 2 schematically illustrates the structure of a thermal barrier coating TBC described by U.S. Pat. No. 5,403,669. This structure differs from the conventional structure of FIG. 1 due to the presence, between the top coat TC and the bond coat BC, of an intermediate diffusion layer DL which is formed by means of the above-mentioned aluminium diffusion process. Said intermediate diffusion layer DL has the property of being able to develop, when exposed to high temperatures (for example above 900° C.), a protective surface film, which substantially consists of Al2O3.
In particular, according to U.S. Pat. No. 5,403,669, it is expedient for the intermediate diffusion layer DL to have at the top a surface roughness Ra (arithmetic mean roughness) between 200 and 600 microinches (that is between 5.08 and 15.24 μm).
Roughness values below 200 microinches (5.08 μm) are not sufficient to guarantee anchoring of the top coat TC. On the contrary, values above 600 microinches (15.24 μm) result in a surface porosity which is too high to be appropriately sealed by the diffused aluminium and consequently opens up preferential routes for premature oxidisation.
Various techniques are available to achieve the diffusion of aluminium inside the bond coat. Of these, an “in pack” method, a method based on chemical vapour deposition (CVD), localised application methods, etc. have been successfully used.
Among the CVD methods, moreover, two different operating modes called “low activity aluminizing” (or also diffusion towards the outside) and “high activity aluminizing” (or also diffusion towards the inside) are possible and widespread.
A “high activity” coating is formed when the activity of the aluminium is greater than the activity of the components of the alloy of which the substrate is composed. The aluminium diffuses towards the inside more rapidly than the speed at which the components of the alloy constituting the substrate diffuse towards the outside. A “low activity” coating is formed, on the other hand, when the activity of the aluminium is less than the activity of the components of the alloy of which the substrate is composed. Typically, the low activity process requires higher operating temperatures. The final structure and composition of the intermediate diffusion layer vary, in general, depending on the type of alloy constituting the substrate.
A typical example of a low density aluminizing process is described e.g. in EP1927672, which teaches applying by cold spray, over a bond coat of CoNiCrAlY alloy, an aluminium film, and subsequently employing a thermal treatment in vacuum in order to favour the diffusion of aluminium into the bond coat.
A thermal barrier coating TBC of the type illustrated in FIG. 2 was applied to turbine components. In the stator portions (generally airfoil) where the only protective coating applied was the one obtained by the aluminizing process, better performance was recorded in terms of resistance to oxidisation and corrosion. Nevertheless, a significant reduction in the surface roughness of the coatings deposited by the “low activity” process when compared with those deposited by the “high activity” process was systematically observed.
The generalised reduction in roughness resulting from a “low activity” aluminizing process of the type described above entails some disadvantages in the areas of the stator where the coating to be applied is of the bond coat/intermediate diffusion layer (Al)/top coat type.
In particular, a sudden deterioration was recorded in the quality of adhesion of the top coat to the layers below of the thermal barrier coating. Furthermore, a significant deterioration was observed in the resistance of the coating to thermal fatigue cycles. Said reductions were identified experimentally by means of comparative thermal cycling tests, and confirmed by comparative tensile strength tests and micrographic analyses. Moreover, the existence of a relationship between resistance of the thermal barrier coating and the degree of adhesion between top coat and layers below has been repeatedly pointed out also in the patent literature (for example in U.S. Pat. No. 4,335,190 and US2007/0178247).
In other words, it has been observed that the application of an aluminium coating by means of a low activity aluminizing process as the one described in the prior art cited is, on the one hand, able to improve the resistance to oxidisation of the portions where the protective aluminium layer is the only one present; at the same time, however, it drastically reduces resistance to thermal fatigue of the thermal barrier coating, where a bond coat is present on top of the aluminium layer. With that method, in fact, a high risk of premature detachment of the top coat has been recorded, with consequent probable functioning problems and significant reduction in the working life of the component.
On average, therefore, a worsening has been observed in the resistance of the turbine component to thermal fatigue.
In order to bring resistance and adhesion of the thermal barrier coating back to values comparable to those consolidated prior to introduction of the low activity aluminizing, a method has been proposed which entails a laborious process of restoring an adequate interface between bond coat and top coat.
In practice, said restoring operation is performed via the deposition of an additional layer of bond coat and subsequent local aluminizing of this additional layer. Following this operation, a roughness Ra greater than 500 microinches (12.7 μm) is recorded (on average).
In this way, optimal roughness is restored at the interface on which the top coat will be deposited. However, the sequence of the layers in the thermal barrier coating is no longer “bond coat/intermediate diffusion layer (Al)/top coat”, but “first bond coat BC/first intermediate diffusion layer (Al) DL/second bond coat BC′/second intermediate diffusion layer (Al) DL′/top coat TC”. Said configuration is shown schematically in FIG. 3 in which, for the sake of simplicity of interpretation, the same reference symbols as those used in FIGS. 1 and 2 have been combined with the corresponding elements.
Firstly, it should be observed that said sequence of layers is not the one generally referred to when producing the construction drawings of the components. Secondly, with this modified sequence of layers, horizontal delaminations frequently occur with length varying from 0.1 to 2 mm. Furthermore, in practice it is not possible to guarantee that the second intermediate diffusion layer DL′ completely covers the first bond coat BC. Consequently, discontinuities very frequently form which can, in use, undermine the fatigue resistance of the thermal barrier coating as a whole.
In short, while restoring the correct adhesion between bond coat and top coat, the solution proposed for tackling the adhesion problem, provides an overall structure which is far from meeting the generally recognised requirements and standards and which has some inherent internal defects. Furthermore, the application of this solution is particularly demanding in terms of production times and costs.
The need is therefore felt in the sector to provide an alternative method for the formation on a substrate of a thermal barrier coating which allows the drawbacks previously described to be overcome.
In greater detail, the need is felt to provide a method for the formation of a thermal barrier coating such as to meet the requirements of the particular conditions of use of the substrate, in particular when the latter is intended to be exposed, in use, to high temperatures and strongly oxidising conditions. Above all, the requirement is for a process that allows a particularly stable adhesion to be obtained between the top coat and bond coat, or more generally with the layers below of the thermal barrier coating. Finally, the need is felt in the art for a method for the formation of a thermal barrier coating such as to ensure improved performances in terms of resistance to thermal fatigue cycles, thereby preserving for as long as possible the mechanical properties of a component protected with said thermal bond coating.